Laser produced plasma EUV light sources have been under discussion for some time. The above referenced co-pending patent application discusses such LPP EUV lights sources and the background relating to them. Other co-pending applications assigned to the common assignee of the present invention also relate to aspects of DPP EUV light sources, including, U.S. patent application Ser. No. 10/798,740, entitled COLLECTOR FOR EUV LIGHT SOURCE, filed on Mar. 10, 2004, Ser. No. 10/900,839, entitled EUV LIGHT SOURCE, filed on Jul. 27, 2004, the disclosures of which are hereby incorporated by reference. The present application relates to improved drive lasers for a LPP EUV light source and improvements in delivering the laser irradiation to the plasma initiation target.
E. Takahashi et al., “KrF laser picosecond pulse source by stimulated scattering processes”, Opt. Commun. 215, 163-167 (2003); K. Kuwahara et al., “Short-pulse generation by saturated KrF laser amplification of a steep Stokes pulse produced by two-step stimulated Brillouin scattering”, J. Opt. Soc. Am. B 17, 1943-1947 (2000); R. Fedosejevs and A. A. Offenberger, “Subnanosecond pulses from a KrF Laser pumped SF6 Brillouin Amplifier”, IEEE J. QE 21, 1558-1562 (1985).
S. Schiemann et al., “Efficient temporal compression of coherent nanosecond pulses in a compact SBS generator-amplifier setup”, IEEE J. QE 33, 358-366 (1997).
H. Nishioka et al., “UV saturable absorber for short-pulse KrF laser systems”, Opt. Lett. 14, 692-694 (1989) and E. Takahashi et al., “High-intensity short KrF laser-pulse generation by saturated amplification of truncated leading-edge pulse”, Opt. Commun. 185, 431-437 (2000) discuss various aspects of Brilluoin scattering and Stokes pulse production schemes that include the use of saturable absorbers but without apertures in SBS cells or the use of SBS amplifier cells. They do not teach or suggest, however, any optimization, e.g., of an energy conversion efficiency, e.g., for use in producing EUV light with short duration drive laser pulses. They also do not teach or suggest, e.g., the use of this method for a drive laser for laser-produced plasmas. Neshev, et al., “SBS Pulse Compression to 200 ps in a Compact Single Cell Setup,” Appl. Phys., B 68 (1999), pp 671-675, discusses single cell and single cell plus an amplifier cell and Stokes pulse production for SBS.
H. Eichler, et al., “Phase conjugation for realizing lasers with diffraction limited beam quality and high average power,” Techninische Universitat Berlin, Optisches Institut, June 1998 web page, discusses:
phase conjugation by SBS (stimulated Brillouin scattering) is a powerfull and simple tool to increase the beam quality of high power lasers up to the diffraction limit. Master oscillator and power amplifier systems with Nd:YAIO as active medium were realized with an average output p0ower up to 210 Watts at 1.08 μm in near diffraction limited beam. Also Nd:YaG-oscillators with phase conjugating mirrors were realized with an average output power up to 17 Watt and high beam quality. In the UV range, excimer oscillators with SBS mirrors deliver the same or better beam quality than conventional plane-plane resonators in 3 times shorter pulses. Excimer oscillator-amplifier arrangements have been realized with SBS mirrors. SBS mirrors for the visible and ultraviolet spectral range are well characterized.                Phase conjugation by SBS is a powerful tool to increase the beam quality of high power solid state lasers up to the diffraction limit. Master oscillator and power amplifier syste3 ms with Nd:YAIO as active medium were realized. A simple single rod amplifier system was optimized to produce an average output power tunable from 4 up to 140 Watt with a beam quality of 1.1 times the diffraction limit. . . . Also Nd:YAG oscillators with phase conjugating mirrors were realized with an average output power up to 17 Watts and high beam quality.        In the UV range, excimer oscillators with SBS mirrors deliver the same or a somewhat better beam quality than conventional plane resonators in 3 times shorter pulses. Excimer oscillator amplifier arrangements have been developed with SBS mirrors. Improvement of the output beam quality requires the use of excimer amplifier discharges without amplitude distortions. SBS mirrors for the visible and ultraviolet spectral range are well characterized can be applied also to other laser types, e.g.,        
Titanium:sapphire of Praseodymium lasers.
Tamper materials have been used in implosion systems and in LPP experiments [In what ways and for what purposes? With droplet targets?] and in inertial confinement fusion, but not as applicants presently propose for LPP EUV light production and specifically for improved CE in such applications.
The performance of a fiber amplifiers, e.g., a compact multimode pumped erbium-doped phosphate fiber amplifiers is discussed in Shibin Jiang, et al., “Compact multimode pumped erbium-doped phosphate fiber amplifiers,” Optical Engineering, Vol. 42, Issue 10 (October 2003), pp. 2817-2820. A fiber amplifier with a small signal net gain of 41 dB at 1535 nm and 21 dB over the full C-band, using an 8-cm-long erbium-doped phosphate fiber excited with a 1-W, 975-nm multimode laser diode is discussed. A theoretical model for the multimode pumped amplifier based on modified rate equations and an effective beam propagation method are also discussed. Also see http://www. npphotonics.com/files/oaa—2002. pdf. Michael R. Lange, et al., “High gain coefficient phosphate glass fiber amplifier,” NFOEC 2003, paper no. 126, also discusses phosphate glass material, with its high solubility for rare earth ions, is an attractive host candidate and explores the physical and dimensional parametrics associated with rare earth doped phosphate glass host material for the purpose of determining its benefits, limitations, and suitability as a compact gain media.
Efficient EUV generation using, e.g., molecular fluorine or excimer gas discharge lasers, e.g., a KrF excimer laser may require giving away significant power output, even from a MOPA arrangement, in order to satisfy requirements for short pulse duration for the LPP EUV light source drive laser. Such laser pulsed may be required to be on the order of perhaps as low as 2-3 ns FWHM, but most likely at least no larger than about 5-6 ns. Because, e.g., the KrF excimer laser has an upper state KrF* of the KrF laser transition on the order of 12 ns, it is difficult to get an output pulse of such short duration and in fact difficult to get much below 15-20 ns with state of the art KrF gas discharge lasers, without having to tolerate a significant loss in output power.
Applicants propose a solution to the above addressed problems and applications of noted existing technologies for the improvement of the production of EUV light by laser produced plasma techniques.